But how closely are cancer and obesity actually related? Does one cause the other? And what does weight have to do with your DNA, anyway?

Cancer and obesity research

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It’s not as easy to research obesity in humans as it might seem. Like a lot of research related to health and diet, studies are mostly observational. This means people do a lot of self-reporting, and any links researchers find between habits and conditions aren’t always as closely related as they may appear.

One 2014 study published in the BMJ, for example, found an association between body mass index and cancer. This does not prove obesity directly causes cancer, or that it caused cancer in some or any of the cases studied. There could be other factors that are playing a bigger part in the development of cancer for the participants in question.

The National Cancer Institute notes, however, that additional research has suggested links between obesity and over a dozen types of cancer. These might include breast cancer, endometrial cancer, pancreatic cancer, breast cancer, colorectal cancer, thyroid cancer, and ovarian cancer.

It’s also been made clear that weight gain increases the risk of many other diseases besides cancer — but which can increase cancer risk. People living with obesity are more likely to develop osteoarthritis, for example, an inflammatory health condition that can increase cancer risk.

Why does obesity increase cancer risk?

There are a handful of possible evidence-based reasons why obesity increases the risk of multiple types of cancer. One of them is that obesity often leads to insulin resistance, a precursor to type 2 diabetes that can pave the way for certain types of hormone-related cancers.

Another is that people living with obesity are more susceptible to developing inflammation throughout the body. Over time, this causes changes as far down as their DNA. They are also more likely to develop conditions such as chronic ulcerative colitis that cause inflammation and increase cancer risk.

The National Cancer Institute also reports that the cells that make up fatty tissue (fat cells) likely cause hormone changes that affect cell growth. These cells may directly or indirectly contribute to the abnormal cell growth associated with cancer.

Why is obesity so dangerous?

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According to the National Institutes of Health, overweight and obesity increase your risk of developing health conditions such as high blood pressure, sleep apnea, heart disease, and fatty liver disease.

Many of these health risks have what are often called “silent” symptoms. This makes them especially dangerous because your body could be sustaining damage gradually over time without you knowing it.

High blood pressure, for example, almost never displays symptoms unless someone’s health is seriously at risk. By the time many people find out they have it, they’ve either just had a heart attack or are dangerously close to having one.

Therefore, it’s equally dangerous to assume you’re healthy despite being overweight because you “feel fine.” Your best bet in trying to prevent diseases such as cancer is to take as many steps as possible toward a healthy lifestyle — even if those steps are small and seemingly ineffective at first.